Clinical psychologist Dr. Meg Jay doesn't subscribe to the theory that your 20s are a throwaway time to just have fun and decide what you want to be when you grow up. While popular media often depicts 20-somethings as aimless wanderers lounging in extended adolescence, the truth, according to Jay, is that your 20s are your defining decade.

In fact, that's the title of her new book,"The Defining Decade: Why Your Twenties Matter—and How to Make the Most of Them Now" and the inspiration for her TED talk, which proclaim that 30 is not the new 20, stressing the importance of that crucial time period post-college, especially when it comes to your career. We got Jay on the phone to get her advice on nine of the most common myths about your 20s, and what you should do instead. If you've ever considered bailing on your job or your "temporary" barista job has stretched to three years, you're going to want to hear this.

Myth 1: Your Twenties Don’t Really Count

Despite their best efforts, 20-somethings have been hindered by the recession and difficult economic climate. From saving money to borrowing money, Gen Y lags behind their parents and grandparents. But that doesn't mean they get to take a time out. “Your 20s are the time to make some moves,” Jay says. “It's a unique, potentially transformative time. It ends up being more important than it feels.”

If you're putting off starting your life... don't. It's time to start making deliberate choices in your job, your city and even your love life to set yourself up for the life you want in your thirties. As Jay said to one young woman in her book who said that her choices before age 30 were just practice: "Consider what part you're rehearsing to play."

Myth 2: You Need to Discover Your Perfect Career, Then Do It

Too many 20-somethings think they need to figure out what they want to be when they grow up before landing an actual job. Instead, says Jay, your 20s are the ideal decade to build what she calls "identity capital"—little bits of experience you collect that coalesce into a solid identity over time.

For example, rather than holding out for your absolute dream job, it's O.K. to take a job that isn’t ideal, as long as there's something about the position that could lead to another, better opportunity down the road. It's also fine if it's something a little unconventional. “I always tell my clients to take the job that's going to make people lean forward and say, ‘Tell me about that!’” she says.

Long before becoming a successful psychologist, Jay was an Outward Bound instructor, which her future interviewers found cool. “I was actually the only person in my graduate school class at Berkeley who didn't go to an Ivy for undergrad, but with Outward Bound in my pocket, I didn't need the Ivy distinction," she says. She too, went through a 20-something period of being "underemployed," but made a point of upping her identity capital by choosing such an interesting part-time gig. If you need to make ends meet as a nanny or barista for a time, fine, but also try to find a way to get more high-profile experiences on your résumé. As she writes in her book, no one will start off an interview with, "So tell me about being a nanny."

Myth 3: You Can Do Anything You Want!

Before you get stars in your eyes, you should be realistic about your skills and goals. “Sometimes people get in their 20s and hear, ‘Oh my God, you can do anything you want in the whole wide world!’” Jay says. “That's overwhelming, and it's not true."

In her book, she discusses psychoanalyst Christopher Bollas' idea of "unthought knowns"—things we know about ourselves but forget or suppress, like a childhood dream of working with animals or an aptitude for physics that was abandoned post–high school. To keep from being overwhelmed by "endless" possibilities, put together a handful of concrete plans you could pursue, based on yourunthought knowns.

Myth 4: If You Find the Perfect Job, It Won’t Ever Feel Like Work

Those pretty Pinterest posters about doing what you love every day? They (unsurprisingly) gloss over an inconvenient truth of life: Work will be work, even if your job is leading kayaking trips or being a fashion designer.

But that’s another black-and-white myth 20-somethings tend to subscribe to: I can either follow my passion and work will be effortless, or I can chain myself to a desk in misery. Yes, there are jobs that are wrong for you, but Jay says the idea of the perfect job is like thinking that once you get married, you’ll live happily ever after without ever having one fight. “In truth, all jobs (and relationships) are work,” she says, but that’s how you grow and acquire new skills.

Myth 5: You Can Get the Job You Want Later

Jay says one of the regrets she hears most often from 30-somethings is that they didn’t explore their professional options enough. “I hear, ‘I wish I had made more of that time, and taken more career risks while I had the chance.’” That’s because employers are willing to take risks on you in return while you’re still young and fresh. “Everyone is eager to help young people who are easy to work with," Jay says. "But by your 30s, employers start to look sideways at employees who are still searching for what they want to do."

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Jay writes in "The Defining Decade" that "in the twenty-first century, careers and lives don't roll off an assembly line. We have to put together the pieces ourselves." Keeping that in mind, start reaching for the pieces you want: Without kids or a mortgage, moving cities or changing companies in pursuit of your career isn't out of the question.

Myth 6: If You Don’t Like Your Job, Quit

The famous Holstee Manifesto poster might read, "If you don't like your job, quit," but Jay disagrees. Before you slap your resignation letter on your boss’s desk, pause to ask yourself these questions:

What is it about this job that makes me unhappy?

Am I still getting something from the experience?

Is there anything about it I did like?

Why did I think I wanted to work here?

Then there are the financial considerations. Do you have an emergency fund? Do you have enough savings to support you during a switch? If not, and if you find that your job isn't providing you with anything beyond a paycheck, it might be time to start saving with a job switch in mind. And what if you hate your job because of one person? Jay has something to say about that too…

Myth 7: If You Have a Bad Boss, Leave and Don’t Look Back

A bad boss isn't always a reason to cut and run. “Bad bosses are some of the most toxic experiences you can have as a 20-something,” Jay concedes... but they can also still provide value if you can stick it out. “If you're getting something from the job that will help you succeed later, stay until you have enough of that, and then leave. Use it to get the next job with the better boss.”

“And, whenever possible, leave on good and gracious terms," Jay advises. "The world is smaller than it seems, and even bad bosses might one day be in the position to help you—or not.”

Myth 8: Job Hopping Is Always Bad

Actually, “It's more than O.K.,” says Jay, to try out a few different jobs and even careers in your 20s. “But be targeted and efficient,” she cautions, “so you can come to some preliminary conclusions by 30." Remember, what differentiates a desirable job hopper from an un-hireable one is having a narrative arc to your résumé—a way to tie all these decisions together into a cohesive story.

“If someone says, ‘What have you been doing the last five years?’ you should be able to explain what you tried, and what you're doing now," says Jay. Careers tend to narrow, in a good way. For example, maybe you know you want to be in a helping career, so you try teaching, but you realize you would rather work one-on-one, so you move to social work. "That’s a good narrative arc," she says. “Your career is very agile and changeable over your adulthood, but it has to make some sense.”

Myth 9: 20-Somethings Are Entitled

Gen Y gets a bad rap, but after all of her work with 20-somethings, Jay doesn't view them as narcissistic or entitled, just inexperienced. “Your bosses aren't your parents or teachers—they don't care about your personal development," Jay explains. "That misunderstanding leads to friction and mistakes.”

"Forward thinking doesn't just come with age," she writes in "The Defining Decade." "It comes with practice and experience." 20-somethings will learn to adjust to the workplace—they just need a real-time chance to prove themselves.

Alden Wicker is an assistant editor and voice behind LearnVest's Facebook page and Twitter account. She has written for The Huffington Post, Refinery29, Narrative.ly, and ElephantJournal.com. She has a B.S. in Business Administration and a B.A. in Communications from Washington and Lee University in Virginia. She has a special interest in living the green life, and writes about living sustainably in the city for her personal blog, CleanHippie.net. Follow Alden on Twitter @aldenwicker.